A well designed, for SEO, Drupal website needs some sort of Analytics. The webmaster needs to know how people get to their websites and the subsequent behavior. This is even more important when an eCommerce is involved. I usually use 2 forms of analytics.

Everybody will tell you to use Google Analytics and I agree with them. This free service from Google is extremely valuable especially if you want to measure the bounce rate (how many visitors leave your website looking only at the landing page) and the average time spent on your website (this is self-explanatory). I add to the mix Statcounter. The latter lets you track, page by page, of all visits to your website. To be totally honest I like Statcounter more.

For both these instruments there are plugins; just go on Drupal Modules Repository and search for them. I assume that you know how to add a module on Drupal; If I get enough requests I will post directions on how to adjust settings. Comments are welcome.